Image processing apparatus and image display apparatus using same

ABSTRACT

An image processing apparatus of the present invention comprising (a) a first signal processing circuit for applying gamma correction to an n-bit (n: a natural number) digital signal inputted as a video signal and for converting the n-bit digital signal into an m-bit (m&gt;n, m: a natural number) digital signal, and (b) a second signal processing circuit for adding a noise signal, which is used for pseudo contour reduction, into the m-bit digital signal from the first signal processing circuit and for outputting a Q-bit (Q: a natural number) digital signal, which is obtained from rounding off a less significant (m−Q) bit (Q≦n) from the m-bit digital signal, to a display section.

FIELD OF THE INVENTION

The present invention relates to an image processing apparatus, in whicha digital signal is inputted as a video signal, for use in a digitalimage display apparatus for displaying a digital image, and an imagedisplay apparatus provided with the image processing apparatus.

BACKGROUND OF THE INVENTION

Conventionally, gamma correction is usually applied to a digital videosignal inputted into a digital image display apparatus in order toimprove display properties to the level of that of Cathode Ray Tube(CRT) apparatus.

However, pseudo contour is caused on the display image when inputtingthe gamma-corrected video signal into the digital image displayapparatus without any treatment, thus resulting in lower displayquality.

Therefore, for example, disclosed in Japanese Unexamined PatentPublication Tokukaihei No. 9-185707 (published on Jul. 15, 1997) istechnology for reducing the pseudo contour due to the gamma correction.The technology employs signal processing technology in which errordiffusion method is applied, in other words, the gamma correction, whichis a vital process for a spatial light modulator and a display apparatuswith linear luminescence properties, is carried out by random additionof a controlled noise signal into the video signal.

However, in the error diffusion method disclosed in the publication, thenoise signal, which is to be added to the video signal, is stored andfed back in order to prevent accumulation of errors. The apparatus iscomplicated due to the additional need for a memory, which is used forstoring the noise signal, and for a circuit to feed back. Consequently,a high cost is a problem.

SUMMARY OF THE INVENTION

An object of the present invention is to provide, at a low cost, animage processing apparatus that may achieve reduction of pseudo contour,which is generated by gamma correction, by using a simple circuitstructure, and to provide an image display apparatus provided with theimage processing apparatus.

The image processing apparatus of the present invention is, for thepurpose of achieving the object, utilized for an image display apparatusthat is provided with display apparatus having display properties of nbit (n: a natural number) for displaying an image, when accepting adigital signal as a video signal, in accordance with the digital signal.The image processing apparatus is provided with a first signalprocessing circuit and a second signal processing circuit. The firstsignal processing circuit carries out the gamma correction for an n-bitdigital signal inputted as the video signal, and converts the digitalsignal into an m-bit (m>n, m: a natural number) digital signal. Thesecond signal processing circuit adds a noise signal, which reduces thepseudo contour, to the m-bit digital signal from the first signalprocessing circuit, then outputs a Q-bit (Q: a natural number) digitalsignal, which is obtained by rounding down a less significant (m−Q) bit(Q≦n) from the m-bit digital signal, to the above display apparatus.

Therefore, the inputted n-bit digital signal is expanded to the m-bitdigital signal after the gamma correction. Next, the noise signal forreducing the pseudo contour is added to the m-bit digital signal, whichis converted into the Q-bit digital signal by rounding down the lesssignificant (m−Q) bit from the m-bit digital signal. Then, the n-bitdigital signal is inputted into the display apparatus of Q-bit displayproperties.

Hence, the digital signal without the bit lack is inputted into thedisplay apparatus. In other words, the display apparatus having 8-bitdisplay properties receives the 8-bit digital signal.

Moreover, a radical change in thickness of the color of adjacent pixelsto be displayed is prevented by adding the noise signal, which reducesthe pseudo contour, into the m-bit digital signal. The m-bit digitalsignal has become a video signal with no or little pseudo contourgeneration.

Next, the less significant (m−Q) bit of the m-bit digital signal with noor little pseudo contour generation is rounded off to convert the m-bitdigital signal into the Q-bit digital signal whose bit number is thesame as the display properties of the display apparatus. The Q-bitdigital signal has become a video signal in which the thickness of thecolor of the adjacent pixels of the image to be displayed is notradically changed, in other words, no pseudo contour is generated.

Therefore, a display image of high quality with no or little pseudocontour may be achieved by inputting this Q-bit digital signal into thedisplay apparatus having Q-bit display properties.

In order to achieve the above object, the image processing apparatus ofthe present invention is provided with a signal processing circuit foroutputting the Q-bit digital signal, which is obtained by rounding downthe less significant (m−Q) bit (Q<m, m & Q: natural numbers) from them-bit digital signal after adding the noise signal for reducing thepseudo contour into the inputted m-bit digital signal.

Because the less significant (m−Q) bit is rounded off after the additionof the noise signal into the m-bit digital signal, it is not the thinout of the less significant (m−Q) bit. Thus, it is possible that thedisplay properties equivalent to m bit are expressed in a pseudo mannerby the Q-bit digital signal.

Set to zero is the average value of the signal level of the noise signalto be added into the m-bit digital signal. Accumulation of errors ofsignals due to the noise signal addition will not be caused by adding anoise signal, which is in the above setting, into the digital signal ina random manner.

This may simplify the apparatus, thus offers a low-cost image processingapparatus, because some apparatus such as a memory and a feedbackcircuit are no longer necessary, which are needed in the conventionalerror diffusion method for preventing the accumulation of errors when anoise signal is added into a digital signal.

For a fuller understanding of the nature and advantages of theinvention, reference should be made to the ensuing detailed descriptiontaken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing a schematic structure of an imagedisplay apparatus provided with an image processing apparatus of thepresent invention.

FIG. 2 (a) is a schematic view showing a schematic structure of a firstsignal processing section provided in the image processing apparatusshown in FIG. 1.

FIG. 2 (b) is a graph illustrating a relation between an input value andan output value in the first signal processing section shown in FIG. 2(a).

FIG. 3 is a graph showing a relation between a signal level andtransmittance of the input signal of the first signal processing sectionshown in FIG. 2.

FIG. 4 is a graph showing display properties of a Liquid Crystal Display(LCD) of a display section provided to the image display apparatus shownin FIG. 1.

FIG. 5 is a graph illustrating a relation between a signal level andtransmittance of the output signal of the first signal processingsection shown in FIG. 2.

FIG. 6 is a schematic view showing a schematic structure of a secondsignal processing section provided in the image processing apparatusshown in FIG. 1.

FIG. 7 is a schematic view showing a schematic structure of a noisegenerating circuit provided to the second signal processing sectionshown in FIG. 6.

FIG. 8 is an explanatory view showing a random noise signal.

FIG. 9 is a histogram showing frequency distribution of levels of thenoise signal shown in FIG. 8.

FIGS. 10 (a) to (d) are graphs showing relations between the inputsignal and the output signals of the signal processing section shown inFIG. 1 in case of color image display. FIG. 10 (a) is a graphillustrating a relation between a signal level of the input signal andthe transmittance of the LCD. FIG. 10 (b) is a graph explaining arelation between a signal level of an R (Red) signal among the outputsignals and the transmittance of the LCD. FIG. 10 (c) is a graph showinga relation between a signal level of a G (Green) signal among the outputsignals and the transmittance of the LCD. FIG. 10 (d) is a graphillustrating a relation between a signal level of a B (Blue) signalamong the output signals and the transmittance of the LCD.

FIG. 11 is a schematic diagram showing an image display apparatus forshowing connection formation of respective circuits for rewriting avalue of the LUT of the image processing apparatus of the presentinvention.

FIG. 12 is an explanatory diagram showing how a video signal and asynchronizing signal fed into driving IC, which is used by a displaysection of the image display apparatus shown in FIG. 11, are inputted.

DESCRIPTION OF THE EMBODIMENTS

Described below is the explanation of one embodiment of the presentinvention. It should be noted that a Liquid Crystal Display (LCD)apparatus will be discussed as the image display apparatus in thepresent embodiment.

The LCD apparatus in the present embodiment is, as shown in FIG. 1,provided with a display section 11 as display means for displaying animage in accordance with a video signal, and an image processingapparatus 12 for processing the video signal in accordance with thedisplay properties of the display-section 11.

The display section 11 includes an LCD 13, which has display propertiesof Q bit (Q: a natural number), that is, has tone gradation of 2^(Q)(the Qth power of 2), and a source driver 14 and a gate driver 15 asdrive means for driving the LCD 13.

The source driver 14, upon receipt of a video signal that has beenprocessed by the image processing apparatus 12, sends a voltage, whichvaries depending on the inputted video signal, to a source electrodeline (not shown) of the LCD 13.

On the other hand, the gate driver 15, upon receipt of synchronizingsignals (Horizontal Synchronizing Signal H, Vertical SynchronizingSignal V) that have been outputted from a synchronizing signalgenerating circuit (not shown), sends a voltage, which varies dependingon the inputted synchronizing signals, to a gate electrode line (notshown).

The image processing apparatus 12 is provided with a first signalprocessing section (a First Signal Processing Circuit) 16 and a secondsignal processing section (a Second Signal Processing Circuit) 17. Thefirst signal processing section 16 has bit converting means that appliesgamma correction to an n-bit (n: a natural number) digital signalinputted as a video signal, and converts the digital signal into anm-bit (m>n, m: a natural number) digital signal, then outputs thedigital signal. The second signal processing section 17 converts them-bit digital signal, which is forwarded from the first signalprocessing section 16, into a Q-bit (Q: a natural number) digital signalby rounding down the m bit below a less significant (m−Q) bit (Q≦n).

Therefore, the bit number Q of the digital signal inputted into thesource driver 14 of the display section 11 is equal or less than n-bitnumber that is inputted in.

As the result, the display quality of display image may be significantlyimproved in the LCD apparatus in the above-mentioned arrangement byavoiding pseudo contour generation, which is caused by the bit lackoccurred at the gamma correction of a video signal.

In the following explanation, the values of those natural numbers,namely n, m, and Q, are supposed to be n=8, m=10 and Q=8, respectively.Thus, the LCD 13 will be described as a display apparatus with displayproperties of Q=8 bits. In the image processing apparatus 12, it issupposed that the first signal processing section 16 converts an n-bit,that is, 8-bit digital signal into an m-bit, that is, 10-bit digitalsignal by expanding bit numbers from 8 (N) to 10 (M) then the secondsignal processing section 17 converts the m-bit, that is, 10-bit digitalsignal to a Q-bit, that is, 8-bit digital signal by rounding down lesssignificant 2 bits (m−Q) from the m-bit digital signal.

The image processing apparatus 12 is discussed below.

To begin with, the signal processing made by the first signal processingsection 16 in the image processing apparatus 12 is explained below,referring to FIG. 2 through FIG. 5. Note that, the LCD 13 of the displaysection 11 is made of liquid crystals of normally white mode, in whichits transmittance is decreased as the applied voltage is increased atthe display electrode.

The first signal processing section 16 is, as shown in FIG. 2 (a),provided with a Look Up Table (LUT) 18 as bit converting means to carryout the gamma correction of an inputted 8-bit digital signal and theconversion of the digital signal into a 10-bit digital signal. In otherwords, the LUT 18 is, as shown in FIG. 2 (b), provided for convertingthe signal level (input value) of the 8-bit digital signal into thecorresponding signal level (output value) of the 10-bit digital signal.

The setting of the output value of the LUT 18 is explained below, withreference to FIG. 3 through FIG. 5.

FIG. 3 is a graph illustrating a relation between the signal level ofthe inputted 8-bit digital signal and the transmittance of the LCD 13under the condition of gamma=2.2, which is commonly used for televisionsystems. FIG. 4 is a graph showing a relation between the voltageapplied to the electrode of the LCD 13 and the transmittance of the LCD13 in order to explain the display properties of the LCD 13. Moreover,FIG. 5 is a graph showing a relation between the transmittance of theLCD 13 and the signal level of the digital signal when the 8-bit digitalsignal is converted into the 10-bit digital signal as shown in FIG. 2(b).

The graph in FIG. 5 is prepared in the following manner. According toFIG. 4, the transmittance is 100% at applied voltage of 1.00 V. Thesignal level of the inputted digital signal in this condition is 1023,while the signal level of the inputted digital signal will be lowered to0 when the transmittance is 0% at applied voltage of 4.50 V. Similarly,the transmittance of 50% at 2.50 V applied voltage gives a signal levelof 584, which is calculated from a formula:(1023−(2.50−1.00)/(4.50−1.00)×1024). Accordingly, the signal level willbe 379 with 20% transmittance at 3.20 V applied voltage, 672 with 70%transmittance at 2.20 V applied voltage, and 818 with 90% transmittanceat 1.70 V applied voltage.

Here, the signal level is indicative of the tone gradation of the videosignal. Therefore, the scale of axis of abscissa in FIG. 3 is 0 to 255because 8 bits have a gradation of 2⁸=256, while that scale in FIG. 5 is0 to 1023 since the gradation is 210=1024 for 10 bits.

FIG. 2 (b) is provided for correlating FIG. 5 and FIG. 3 obtained in themanner mentioned above. For example, with 50% transmittance, the inputlevel is 192 according to FIG. 3, while the value for 10 bits is 584according to FIG. 5. Thus, FIG. 2 (b) is the graph for converting theinput with the input value of 192 to the output value of 584. Similarly,the graph shows the conversion of other values as follows: from inputvalue 128 to output value 379 at 20% transmittance, from input value 220to output value 672 at 70% transmittance and from input value 242 tooutput value 818 at 90% transmittance.

FIG. 2 (b) is prepared by calculating the correlation for all the levelsin-this manner.

The digital signal, which was inputted in 8 bits, is converted into the10-bit digital signal by using the LUT 18 where its output value hasbeen determined in the way discussed above. A relation between thesignal level of the 10-bit digital signal after the conversion and thetransmittance of the LCD 13 gives the graph shown in FIG. 5.

Described below is an explanation on the signal processing of the secondsignal processing section 17 in the image processing apparatus 12.

As shown in FIG. 6, the second signal processing section 17 is providedwith a Bit-Depth Extension (BDE) 22 that is composed of a noisegenerating circuit 19 for generating a noise signal to be added into aninputted digital signal, an adding circuit 20 for adding the noise intothe inputted digital signal forwarded from the noise generating circuit19, and a bit number converting circuit 21 for converting the bit numberof the digital signal in which the noise signal is added. Synchronizingsignals (Horizontal Synchronizing Signal H, Vertical SynchronizingSignal V), which are outputted from a synchronizing signal generatingcircuit (not shown), are inputted into the noise generating circuit 19.

Thus, the second signal processing section 17 converts the 10-bitdigital signal, which is outputted from the first signal processingsection 16, into an 8-bit digital signal by adding the noise signal androunding down its less significant 2 bits from the 10-bit digitalsignal.

In the second signal processing section 17, by adding the noise signalinto the digital signal for reducing the pseudo contour, it can beavoided to radically change the thickness of color of pixels adjacent tothe pixels on which the image is to be displayed. Thus, the 10-bitdigital signal with the noise signal addition has become a video signalwith rare occurrence of the pseudo contour.

Next, the 10-bit digital signal with the rare occurrence of the pseudocontour is converted into an 8-bit digital signal after rounding downits less significant 2 bits from the 10-bit digital signal, theninputted into the display section 11 with 8-bit display properties. The8-bit digital signal, which is inputted into the display section 11, hasbecome a video signal without a radical change in the thickness of colorof the pixels adjacent to the pixels on which the image is to bedisplayed. In short, it has become a video signal free from the pseudocontour generation.

Therefore, a display image of high quality with no or little pseudocontour can be achieved by inputting the 8-bit digital signal into thedisplay means with 8-bit display properties.

The noise generating circuit 19 is provided for producing a noise signalto prevent the generation of the pseudo contour in the video signal.

The average value of the signal level of the generated noise signal isset to zero.

Alternatively, a noise can be generated by using a specific patterntable, as described below. The noise generating circuit 19 is composedof a noise ROM 41 and an address counter 42, as illustrated in FIG. 7.The address counter 42 has received the horizontal synchronizing signalH, the vertical synchronizing signal V, and clock (not shown).

A noise enough for one screen is stored in the noise ROM 41, while ahorizontal address is incremented for each horizontal pixel, and avertical address is incremented for each vertical line in the addresscounter 42, where its output is added to the noise ROM 41. As theresult, a noise for one screen is consecutively supplied per pixel fromthe noise ROM 41. The noise is added to the video signal by the addingcircuit 20 shown in FIG. 6.

The noise ROM 41 may be arranged so that the noise is stored per smallblock such as 16×16 or 32×32 size, instead of one screen. In this case,the address counter 42 is adapted for being reset every 16 or 32 inorder that the same noise data are repeatedly outputted per block. Thehorizontal synchronizing signal H and the vertical synchronizing signalV, which are added to the address counter 42, are used for this purpose.In addition, a problem that periodicity is visualized will not occureven when the noise data are dealt per block such as 16 or 32, becausethe noise data are a random signal as explained later. The abovearrangement gives such an advantage that ROM capacity of the noise ROM41 may be significantly cut down.

Moreover, the address counter 42 may be adapted as described below. Inshort, the section may be adapted so that leading address is altered foreach field of a screen (each vertical synchronizing signal V) or foreach block border.

For example, when a noise is transformed into a 16×16 block structure,where the read-out of noise is carried out consecutively per pixel, in afirst block, the read-out starts from address (0,0) of the noise ROM 41and is continued until address (15,15). In a following block, it isdesigned that the read-out is performed from address (1,1) of the noiseROM 41 to address (15,15), then, through addresses (0,0) and (0,15),continued until address (1,0). In a next block, starting from address(2,2), the read-out is continued until address (15,15), then, viaaddresses (0,0), (0,15), (1,0) and (1,15), ends at address (2,1). Theremaining will be read out in the same fashion.

In this example, the starting addresses of both of the horizontal andthe vertical ones, are incremented by one address per block border, inother words, for each read-out of one block. But it is not limited tothis example. For example, more than two addresses may be incremented,or either of the horizontal or the vertical one may be incremented.

Alternatively, when the leading addresses are changed per frame, wherethe read-out of noise is carried out consecutively per pixel, in thefirst frame, the read-out starts from address (0,0) of the noise ROM 41and is continued until address (15,15). Similarly, in the followingframe, the read-out is also carried out consecutively from address(0,0). The read-out in this frame is repeated consecutively in thesubsequent series of read-outs. In the next frame, it is designed thatthe read-out, starting from address (1,1) of the noise ROM 41, iscontinued until address (15,15), then, through addresses (0,0) and(0,15), continued until address (1,0). The read-out in this frame iscontinued in this manner until the end. In the further next frame,starting from address (2,2), the read-out is continued until address(15,15), then via addresses (0,0), (0,15), (1,0) and (1,15), is ended ataddress (2,1). The remaining will be read out in the same manner.

In this case, as to both of the horizontal and the vertical addresses,the starting addresses are incremented by one address per frame. But itis not limited to this case. For example more than two addresses may beincremented, or either of the horizontal or the vertical may beincremented.

Discussed above are the methods for performing the read-out per block inthe frame and altering the starting address, and the method for carryingout the read-out per frame and altering the starting address. However,other similar combinations may be thought of for convenience. The blockborder, which is originally inconspicuous, may be made furtherinconspicuous by altering the read-out starting address consecutively inthe above manner, because the block structure of the noise, which has ablock structure such as 16×16, is vanished by using this kind ofread-out method.

Moreover, especially for the method in which the starting position isaltered per frame, the following specific effects are produced. An LCDapparatus is generally unable to respond to a speed faster than a speedcorresponding to one frame, thus cannot perfectly follow a noise changedper frame. As a result, the value obtained from the sum of a signalvalue and the average value of the noise is displayed per pixel. Forthis reason, errors, which are equivalent to those errors when allpixels are rounded down evenly, may be generated in this method, evenwith a process for rounding down a signal by some bits as discussedlater. This means that all pixels may express all the quantizationlevels.

On the contrary, in the method previously explained where a noise is notaltered per frame, the tone gradation is expressed in so-called theprinciple of area tone gradation. Therefore, when the respective pixelsare considered individually, it cannot be said that all the quantizationlevels are expressed for the respective pixels. Thus, the method may becalled as a method to express all the tone gradations on average inaccordance with the size of a screen.

Accordingly, in the method in which a noise is changed per frame, allthe tone gradations may be expressed per pixel. If this method isapplied together with a method where leading addresses are altered perblock, as described previously, it produces some properties in which theblock border is inconspicuous even with little memory capacity.Furthermore, as discussed later, extra properties that are suitable forliquid crystals may also be created by devising the properties of thenoise.

This prevents the accumulation of average errors in the digital signal,to which the noise signal generated by the noise generating circuit 19is added, and eliminates the need of a special circuit for theprevention of the error accumulation.

Note that, to make zero the average value of the signal level in thegenerated noise signal may be easily achieved, for example, by utilizingrandom number generating software and algorithm by which the average ofthe amplitude of the signal of the generated random numbers may becomezero.

The random number generating software is used for generating a randomnoise signal. The noise signal preferably has a low amplitudeauto-correlation between adjacent pixels, and has no amplitude value ofzero in its distribution range. Also, the histogram of the amplitude ofthe noise signal preferably shows Gaussian distribution in which thezero amplitude of the noise signal is at the center.

Therefore, the random noise signal has such characteristics that, asshown in FIG. 8, the amplitude of noise level ΔV will never exceed theupper and lower limits of the level and the average value of the levelis always zero within a predetermined range (for example, one field).The histogram of noise level is, as shown in FIG. 9, regarded that it isin its most natural state when it shows the Gaussian distribution.

If the noise signal has low auto-correlation between adjacent pixels,the noise signal to be added has no periodicity but high randomness inthe cycle of its amplitude. This is the point where the presentinvention is effective especially when used for an LCD apparatus. Dotreversing, line reversing or frame reversing drives are generallycarried out in an LCD apparatus in order to perform AC drive. However,different LCDs have different reversing cycles. Thus, the cycle ofreversing is not uniformed.

Those reversing drives cause killer pattern in LCDs. In a rare case, itis required to display the killer pattern, and a significant degradationin image quality will be caused. Therefore, not all images can bedisplayed in a satisfactory image quality. In addition, because thereversing drive is carried out based on an artificial regular cycle, thekiller-pattern-affected image is likely given a regular cycle.

In a rare case, its regularity is interfered with that of the reversingdrive, and an added noise signal with regular cycles of the amplitudemay cause a significant image degradation. For instance, while an LCDapparatus has no such problem, other LCD apparatus may be suffered fromthe problem. For most of such cases, the problem is so severe that notonly image quality is difficult to be improved but also the cause ishard to be found.

On the contrary, use of an irregular noise signal as the added signallike the present invention causes no interference with the regularity ofthe reversing drive, thus causing no image degradation. In addition, nokiller pattern is produced for any LCD apparatus, thus causing no imagedegradation.

Moreover, several advantages are given by the noise signal with zerovalue of the average value of the amplitude, and explained below. Inthis circuit, since noise signals with different values should be addedto each pixel in order to reduce the pseudo contour, it is required tochange the values corresponding to the added noise signals for eachpixel. But, it will be a problem that the brightness of the screen ischanged for a large area. The brightness of the screen for such a largearea changes by the amount corresponding to the average value of theamplitude of the noise signal when adding of the noise signal.Accordingly, the average value of the noise signal should be zero inorder that the brightness may not change during the addition of thenoise signal. Therefore, only the reduction of the pseudo contour isachieved by setting to zero the average value of the amplitude of thenoise signal, without undesirable changes in the brightness in a largearea of the screen.

Furthermore, since the noise signal has a histogram in which no zerovalue of the amplitude exists in the distribution range and which is theGaussian distribution with the zero value at the center, the pseudocontour may be vanished irrespective of conditions even when a videosignal is rounded down to certain bits. On the other hand, there aresome cases that the zero value of the amplitude may cancel out theeffect of noise signal when the value is rounded down.

Another advantage is that the noise of the display image becomes leastvisible when the histogram of the amplitude of the noise signal is inGaussian distribution. On the contrary, without the Gaussiandistribution, the visibility of the noise itself degrades the imagequality.

Furthermore, an image processing apparatus with low cost may be provideddue to the simplified structure by cutting down a memory and a feedbackcircuit, needed in the conventional error distribution method, forpreventing the error accumulation during the addition of a noise signalinto a digital signal.

The 8-bit digital signal outputted from the second signal processingsection 17 is inputted into the source driver 14 of the display section11 shown in FIG. 1. Then, after conversion of voltage into apredetermined level by the source driver 14, the digital signal is sentto a signal line in the LCD 13.

Thus, the pseudo contour due to the bit lack will be eliminated sincethe 8-bit video signal is inputted into the display section 11 having8-bit display properties without the bit lack. Furthermore, the pseudocontour generated by gamma correction is also reduced by adding a noisesignal during the process in order to reduce the pseudo contour. As theresult, an image with high display quality is achieved.

Note that, in the LCD apparatus in the above arrangement, it is alsopossible to have three sets of the first signal processing section 16and the second signal processing section 17 shown in FIG. 1 for RGBcolors when color images are displayed.

For instance, when a video signal with such signal level shown in FIG.10 (a) is inputted into the first signal processing sections 16, theLUTs 18 provided for the respective RGB colors in the first signalprocessing sections 16 give signals in accordance with the respectiveRGB colors as shown in FIG. 10 (b) to FIG. 10 (d). The LUTs 18 may beadapted to carry out the gamma correction so that, when the input signalis an 8-bit signal, the output signal is a 10-bit signal, just like thecase described previously. It is expected that there are somedifferences among the properties of the respective RGB colors.

In this case, the gamma-corrected output signals of the respective RGBcolors are inputted into the second signal processing sections 17, andoutputted as 8-bit signals into the display sections 11 after the bitconversion process.

Moreover, because the display quality of the LCD apparatus is adjustableby the image processing apparatus 12, the image processing apparatus 12and the display sections 11 may be separately provided. In this case,even with some differences among the display sections 11, because thedisplay quality can be adjusted properly by the image processingapparatus 12, any types of display sections 11 may be used for achievingan image of high display quality.

For example, when the LCD apparatus having the above arrangement isadapted to a display apparatus such as a personal computer (PC) whichcontrols the display by a video card, this may improve the image qualityby using an existing PC with a low-cost video card only, but not byusing an expensive video card, because the high quality image isachieved not by the video card but by the image processing apparatus 12provided to the LCD apparatus.

By the way, the LUT 18 included in the image processing apparatus 12having the above arrangement, as described above, is arranged inaccordance with the display properties of the display section 11. But,it is also possible that the LUT 18 is adapted to use a present value orto be rewritable in real time as described below.

Described below is the explanation on the method of rewriting the LUT18, giving concrete examples of the LUT 18 rewriting methods.

1. rewriting in accordance with the display properties (V-T curve) ofthe LCD.

2. rewriting so as to absorb the unevenness between the respective ICs(source).

3. rewriting the bit distribution in accordance with the brightness insurroundings.

4. rewriting the bit distribution in accordance with the average levelof an input signal.

5. preparing three LUTs 18 for the respective RGB colors, and rewritingthem with adjustment for the respective colors.

The apparatus for realizing the above methods is something like an imagedisplay apparatus shown in FIG. 11. Note that, identical members withthe ones used in the previous explanation are assigned, thus theirexplanation is omitted here.

The image display apparatus mentioned above is provided with, as shownin FIG. 11, a display section 11 and an image processing apparatus 32that includes an LUT 18, a BDE 22, a control circuit 23, an area judgingcircuit 24, an average value calculating circuit 25 and a memory section26. The display section 11 has the same arrangement as the image displayapparatus shown in FIG. 1.

The area judging circuit 24 is adapted to receive a synchronizing signalidentical with the one inputted into the source driver 14 and the gatedriver 15, and to judge the area to be displayed on the LCD 13. Then,the area judging circuit 24 is adapted to output the judged areainformation to the control circuit 23.

The average value calculating circuit 25 is adapted to receive a videosignal identical with the one inputted into the LUT 18, and to calculatethe average value of the brightness included in the video signal. Then,the average value calculating circuit 25 is adapted to output thecalculated value to the control circuit 23.

Moreover, the image display apparatus is provided with a brightnesssensor 27 for detecting the brightness in the surroundings of the imagedisplay apparatus and a user's adjusting circuit 28 for allowing a userto alter the LUT 18. Signals from the respective sensor and circuit areinputted into the control circuit 23.

Furthermore, a signal can be inputted from a PC 41 through an interface40. In this case, it is an advantage that a user is allowed to giveinstructions from the PC 41.

As described, the control circuit 23 in the image processing apparatus32 is adapted to receive signals from the area judging circuit 24, theaverage value calculating circuit 25, the brightness sensor 27, theuser's adjusting circuit 28, and the interface 40, and to calculate theLUT 18 separately based on the respective signals. The calculated valueis outputted to the memory section 26 connected with the control circuit23. Note that, each input may include all of or only part of them.

The memory section 26 is provided with a ROM and a RAM. Written inadvance in the ROM is a value for correcting the V-T curve of the LCD 13in accordance with the display properties of the display section 11,because current data cannot be written into the ROM. The RAM is adaptedfor storing the value from the control circuit 23 since it is datarewritable.

If necessary, the stored value for rewriting the LUT 18 is outputted asthe LUT 18 from the memory section 26 by the control circuit 23. Thismakes the LUT 18 rewritable in various conditions.

The high quality of display image is always maintained by rewriting theLUT 18 in accordance with the various conditions as explained above.

Described below is the explanation about the rewriting method of the LUT18 for cancelling out the unevenness among the respective ICs, withreference to FIG. 11 and FIG. 12.

As shown in FIG. 12, the source driver 14 is composed of a plurality ofsource driving ICs 29, while the gate driver 15 includes a plurality ofgate driving ICs 30.

The source driving ICs 29 are connected with a predetermined number ofsource signal lines (not shown) of the LCD 13, while the gate drivingICs 30 are connected with a predetermined number of gate signal lines(not shown) of the LCD 13. For instance, when the LCD 13 has 800×600resolution and is provided with eight of the source driving ICs 29 andsix of the gate driving ICs 30, the number of source signal linesconnected to one source driving IC 29 is one hundred, while the numberof gate signal lines connected to one gate driving IC 30 is also onehundred.

Further, a synchronizing signal is inputted into the source driving ICs29 and the gate driving ICs 30. Hence, detected by the synchronizingsignal is the area surrounded with the respective source driving ICs 29and gate driving ICs 30, in other words, an area 13 a surrounded with100×100 signal lines.

Those areas are driven by specific driving ICs, and have a problem thatunevenness in the properties of driving ICs cause, for example,perceptible differences in luminance among the respective areas. Thedifferences in luminance among the respective blocks, in a sense, areequivalent to the pseudo contour discussed in the present invention.Therefore, the unevenness in screen display properties, which areoriginally due to the unevenness in the driving ICs, may be reduced byusing the present invention that is effective to suppress the pseudocontour. Moreover, the unevenness in the properties of the respectiveareas can be completely suppressed in such an arrangement that theunevenness in the properties of the respective areas 13 a of the LCD 13are detected in advance.

In the areas 13 a detected by using a synchronizing signal as describedabove, the display properties are checked in advance by a shipping-checkapparatus, in order to detect the unevenness in the properties of thedriving ICs (the source driving ICs 29 and the gate driving ICs 30). Theunevenness in the driving ICs is stored in the memory section 26. Then,the LUT 18 is rewritten so that the unevenness in the drive ICs isabsorbed. This absorbs the unevenness in the display properties amongthe respective display areas generated in the display section 11, thusabsorbing the unevenness in the display properties of the respectivedisplay sections 11. Thus, the unevenness between each image displayapparatus can be eliminated.

In practice, written into the LUT 18 is a value in accordance with theproperties of the source driving ICs 29 corresponding to the area of thedisplay section 11, the area being judged by the area judging circuit24. In this case, the LUT 18 is consecutively rewritten in time seriessince the video signal is inputted in time series into the sourceoperative ICs 29. The display quality of the display image of thedisplay section 11 is improved by consecutively rewriting the LUT 18 inthis manner.

Note that, it is also possibly arranged that the LUTs 18 are provided inthe number as many as the source driving ICs 29, and values inaccordance with the respective source driving ICs 29 are written intothe LUTs 18 in advance. In this case, it is also possible to provide theLUTs 18 themselves in the source driving ICs 29.

Moreover, the display image differently comes in sight depending on thesurroundings during the image displaying. In this case, the LUT 18 maybe rewritten so that, when, for example the image display apparatus isplaced in a room, the surroundings of the image display apparatus arebright, the tone gradation of the bright part of the image displayed onthe display section 11 is made to be high, and when the surroundings aredark, the tone gradation of the dark part of the image displayed on thedisplay section 11 is made to be high. In other words, the LUT 18 may berewritten so that the bit distribution of the display image is rewrittenin accordance with the brightness in the surroundings of the imagedisplay apparatus.

In practice, it may be arranged that the brightness in the surroundingsin which the image display apparatus is placed is detected by utilizingthe brightness sensor 27. Then, in accordance with the detection signal,the control circuit 13 sets a suitable value and then rewrites the LUT18 via the memory section 26.

As described above, the image displayed on the display section 11 maybecome more easy to watch by rewriting the LUT 18 in accordance with thebrightness of the surroundings in which the image display apparatus islocated.

Moreover, the display image also becomes hard to watch in case of anoverall dark image or an overall bright image, regardless of thesurroundings of the image display apparatus. In order to solve this, itmay be arranged that the average value of the signal level of theinputted video signal is calculated and the LUT 18 may be rewritten sothat the bit distribution of the display image is rewritten to theaverage value.

In practice, the procedures are as follows: the average value of thesignal level of the video signal is calculated by the average valuecalculating circuit 25. The control circuit 23 sets a suitable value inaccordance with the average value and rewrites the LUT 18 via the memorysection 26. In other words, the LUT 18 may be rewritten so that the bitdistribution is rewritten in accordance with the average value of thesignal level of the video signal that is an inputted signal.

Furthermore, in case of the color display image as described previously,three units of the LUTs 18, which are data rewritable in real time inaccordance with various conditions as described above, may be providedfor the respective RGB colors, and are adjusted to the respective colorsfor rewriting.

Note that, the example in the present embodiment is that an 8-bitdigital signal is expanded to 10 bits and converted back to 8 bitsagain. However, the present invention is not limited to the foregoing,and it may be applied to, for example, where a 10-bit signal isdisplayed on a display of 10-bit display properties, or where a 4-bitsignal is displayed on a display of 4-bit display properties, which isgenerally used in game machines or portable phones.

As discussed earlier, the image processing apparatus of the presentinvention is utilized for an image display apparatus provided withdisplay means of n-bit (n: a natural number) display properties fordisplaying an image, upon receipt of a digital signal as a video signal,in accordance with the digital signal. The image processing apparatus isprovided with a first signal processing circuit and a second signalprocessing circuit. The first signal processing circuit carries outgamma correction for an n-bit digital signal inputted as the videosignal, and converts the digital signal into an m-bit (m>n, m: a naturalnumber) digital signal. The second signal processing circuit adds anoise signal for reducing pseudo contour, to the m-bit digital signalfrom the first signal processing circuit, then outputs a Q-bit (Q: anatural number) digital signal, which is obtained by rounding down aless significant (m−Q) bit (Q≦n) from the m-bit digital signal, to theabove display means.

Accordingly, a display image of high quality with little or nooccurrence of the pseudo contour may be achieved by inputting an n-bitdigital signal into display means with display properties of bit numberless than or equal to n bit.

The image processing apparatus of the present invention is provided witha signal processing circuit for outputting the Q-bit digital signal,which is obtained by rounding down the less significant (m−Q) bit (Q<m,m & Q are natural numbers) from the m-bit digital signal after addingthe noise signal for the reduction of the pseudo contour into theinputted an m-bit digital signal.

Therefore, it is not simply to omit the less significant (m−Q) bit, butto achieve the expression of the display properties equivalent to m bitin a pseudo manner by using a Q-bit digital signal.

The noise is produced so that the average value of the signal level ofthe noise signal to be added into the m-bit digital signal is set tozero.

Accumulation of errors of signals caused by the noise signal additionwill not be resulted from adding a noise signal produced as explainedabove into the digital signal in a random manner.

This may simplify the apparatus, thus offers a low-cost image processingapparatus because means such as a memory and a feedback circuit are nolonger necessary, which are needed in the conventional error diffusionmethod for preventing the accumulation of errors when a noise signal isadded into a digital signal. In addition, it is also possible to includethe entire circuit in the source driving IC for greater economiceffects.

The noise signal may have randomness with no regularity in the cycle ofthe amplitude.

In this case, because no regularity of the cycle of the amplitude of thenoise signal interferes with the regularity of reversing drives in casethat an LCD apparatus is used as the image display apparatus, thedisplay quality of the display image may be improved, accordingly.

The noise signal may, using an arbitrary noise pattern table, beobtained by switching the starting point of the noise pattern table perfield or per noise pattern table.

In this case, because the noise level is evenly rounded down byswitching the starting point of the noise pattern table per field, allpixels are allowed to express all the values of the quantization levels.

Moreover, the quantization levels may be averaged per noise patterntable by switching the starting point of the noise pattern table pernoise pattern table.

In an LCD apparatus, dot reversing, line reversing, or frame reversingdrives are generally carried out for AC drive, but the cycle of thereversion is different for every LCD apparatus and is not uniformed. Dueto this reversing drive, a killer pattern may exist in case there isregularity in the cycle of the amplitude of the noise signal. In a rarecase where the killer pattern is displayed, a significant degradation inimage quality is caused. However, if there is no regularity in the cycleof the amplitude of the noise signal, as described above, a killerpattern will never exist and cause no degradation in image quality.

The histogram of the noise signal may be in Gaussian distribution wherethe zero value of the noise signal is set at the center.

In this case, because the histogram of the amplitude of the noise signalis in the Gaussian distribution, the noise becomes least visible, as theresult, the display quality of the display image can be improved.

The first signal processing circuit may be provided with bit convertingmeans for converting the inputted n-bit digital signal into the m-bitdigital signal in accordance with a value set in advance.

In this case, the display quality of the display image can be furtherimproved by setting the value to be set in the bit converting means,considering various conditions such as the display properties of thedisplay means or the environment surrounding the image displayapparatus.

For example, the value set in the bit converting means may be rewrittenso as to absorb the unevenness in the properties of the drive means fordriving the display means.

In this case, the unevenness among individual display means need not beconsidered so strictly, because the unevenness in the display propertiesgenerated for every display means can be absorbed. As the result, theoverall yield of the image display apparatus is improved, thussignificantly reducing the manufacturing cost of the image displayapparatus.

Besides, the value thus set by the bit converting means may be rewrittenin accordance with the brightness in the surroundings of the imagedisplay apparatus.

In this case, the image can be displayed in high quality all the time,regardless of the brightness of the surroundings of the image displayapparatus, by rewriting the value so that the tone gradation of darkparts of the display image is made to be high when the surroundings ofthe image display apparatus are dark, while, when the surroundings ofthe image display apparatus are bright, by rewriting the value so as tomake that of bright part to be high.

Furthermore, the value set by the bit converting means may be rewrittenin accordance with the display properties of an image displayed on thedisplay means.

In this case, the image can be displayed in high quality all the time,regardless of the brightness of the image displayed on the image displayapparatus, by rewriting the value so as to make the tone gradation ofdark parts of the display image to be high when the entire display imageis dark, while, when the entire display image is bright, by rewritingthe value so as to make that of bright part to be high.

The bit converting means may be realized by an LUT for replacing theinputted signal with corresponding output value set in advance.

In this case, the process in the bit converting means may be promptlycarried out because the output value varying depending on with theinputted value is set in advance in the LUT. Additionally, the entirearrangement of the image processing apparatus may be streamlined bysimplifying the structure of the bit converting means.

Also, the bit converting means may be realized by a calculating devicefor converting an n-bit digital signal into an m-bit (m>n) digitalsignal by numerical calculation.

In this case, the calculating device can be a Central Processing Unit (aCPU) or a Digital Signal Processor (a DSP).

Those CPU and DSP, which are programmable devices, have an advantage tofacilitate user interface for rewriting the value of the bit convertingmeans, thus making the operation easier.

When a color image is displayed on the image display apparatus, therespective RGB colors may be provided with the first signal processingcircuit and the second signal processing circuit.

Furthermore, the image display apparatus may be provided with the imageprocessing apparatus of the above arrangement.

In this case, the process of the signal inputted into the displaysection of the image display apparatus is carried out in the imageprocessing apparatus. This eliminates the need of an expensive apparatusfor improving the display quality of the display section, thus providesthe image display apparatus of high quality in a low cost.

The invention being thus described, it will be obvious that the same waymay be varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are intendedto be included within the scope of the following claims.

1-16. (canceled)
 17. An image processing method comprising the steps of:converting an n-bit (n: a natural number) digital signal inputted as avideo signal, into an m-bit (m>n, m: a natural number) digital signal;adding a noise signal to the m-bit digital signal; rounding down a lesssignificant (m−Q) bit (Q≦n, Q: a natural number) from the m-bit digitalsignal having added thereto the noise signal; and outputting theresulting Q-bit digital signal.
 18. The image processing method as setforth in claim 17, wherein: the inputted n-bit digital signal isconverted into the m-bit digital signal in accordance with a pre-setvalue.
 19. The image processing method as set forth in claim 17,wherein: an average value of a signal level of the noise signal is setto zero.
 20. The image processing method as set forth in claim 17,wherein: the noise signal is a random noise signal with no regularity inits cycle of amplitude.
 21. The image processing method as set forth inclaim 17, wherein: the noise signal is generated for each frame.
 22. Theimage processing method as set forth in claim 17, wherein: the noisesignal is generated for each predetermined block which is smaller thanone frame.
 23. An image processing method comprising the steps of;adding a noise signal to an m-bit digital signal (m: a natural number)inputted as a video signal; rounding down a less significant (m−Q) bit(Q<m) from the m-bit digital signal having added thereto the noisesignal; and subsequently outputting the resulting Q-bit digital signal.24. The image processing method as set forth in claim 23, wherein: anaverage value of a signal level of the noise signal is set to zero. 25.The image processing method as set forth in claim 23, wherein: the noisesignal is a random noise signal with no regularity in its cycle ofamplitude.
 26. The image processing method as set forth in claim 23,wherein: the noise signal is generated for each frame.
 27. The imageprocessing method as set forth in claim 23, wherein: the noise signal isgenerated for each predetermined block which is smaller than one frame.28. The image processing method as set forth in claim 18, wherein: anaverage value of a signal level of the noise signal is set to zero. 29.The image processing method as set forth in claim 18, wherein: the noisesignal is a random noise signal with no regularity in its cycle ofamplitude.
 30. The image processing method as set forth in claim 18,wherein: the noise signal is generated for each frame.
 31. The imageprocessing method as set forth in claim 18, wherein: the noise signal isgenerated for each predetermined block which is smaller than one frame.32. The image processing method as set forth in claim 24, wherein: thenoise signal is generated for each frame.
 33. The image processingmethod as set forth in claim 24, wherein: the noise signal is generatedfor each predetermined block which is smaller than one frame.
 34. Theimage processing method as set forth in claim 25, wherein: the noisesignal is generated for each frame.
 35. The image processing method asset forth in claim 25, wherein: the noise signal is generated for eachpredetermined block which is smaller than one frame.